The invention relates to a method of wire bonding and especially, a method of forming wire bonds on electronic devices using an ultrasonic transducer.
Ultrasonic wire bonding is used to attach electrically conducting wires between contact pads on a semiconductor die and contact areas on a substrate, such as a leadframe, to which the die is attached prior to the electronic device and substrate assembly being encapsulated.
Conventional ultrasonic transducers for wire bonding comprise a piezoelectric driver/motor which is coupled to one end of an ultrasonic vibration amplifying horn. The amplifying horn includes a mounting structure (commonly known as a barrel or a collar) which permits the transducer to be mounted on a wire bonding machine. Mounted at the opposite end of the amplifying horn from the piezoelectric driver/motor is a bonding tool, commonly known as a capillary, through which the wire to be bonded extends during a wire bonding operation. Therefore, the mounting structure is located between piezoelectric driver/motor and the bonding tool.
As the mounting structure is offset from the piezoelectric driver/motor, in order to obtain the most efficient ultrasonic bonding, a zero amplitude of vibration (or node) should occur at the piezoelectric driver and at the point of contact between the mounting structure and the wire bonding machine, and a maximum amplitude of vibration should occur at the bonding tool.
In practice, due to these constraints, it has only been possible to operate this conventional type of ultrasonic transducer at a single frequency which is the design frequency specified by the manufacturer of the transducer. If it is desired to use a different frequency for the bonding operation, then it is necessary to replace the transducer with another transducer with a different specified design frequency.
In order to overcome the limitations of these conventional ultrasonic transducers, U.S. Pat. No. 5,578,888 proposes an ultrasonic transducer which is symmetrical about the mounting axis and has a piezoelectric driver with a central axis that is coincident with the mounting axis. Designing the transducer with the piezoelectric driver coincident with the mounting axis enables the transducer to be operated at more than one resonant frequency to perform wire bonding operations at difference frequencies.
However, a disadvantage of the transducer proposed in U.S. Pat. No. 5,578,888 is that it requires a specially designed and constructed transducer.
In accordance with a first aspect of the present invention, a method of bonding wires between a semiconductor die and a substrate on which the die is mounted comprising providing an ultrasonic transducer comprising an ultrasonic energy generation device, an ultrasonic vibration amplifying device coupled at one end to the ultrasonic wave generation device, a bonding tool coupled to the opposite end of the ultrasonic vibration amplifying device, and a mounting structure located between the ends of the ultrasonic vibration amplifying device; performing a first wire bond by operating the ultrasonic transducer at a first ultrasonic frequency and performing a second wire bond by operating the ultrasonic transducer at a second ultrasonic frequency, the second ultrasonic frequency being different from the first ultrasonic frequency.
Preferably, the method further comprises identifying two ultrasonic frequencies of vibration of the energy generation device at which the vibration amplitude is:
(i) substantially zero at an energy generation plane, the energy generation plane being perpendicular to a longitudinal axis extending between the ultrasonic generation device and the bonding tool and the energy generation plane intersecting the ultrasonic generation device;
(ii) substantially zero at a mounting plane, the mounting plane being perpendicular to the longitudinal axis and being coincident with a mounting contact point between the mounting structure and a wire bonding machine on which the transducer is mounted; and
(iii) non-zero at a bonding plane, the bonding plane being perpendicular to the longitudinal axis and intersecting the bonding tool.
Preferably, the vibration amplitude is substantially a maximum at the bonding plane.
Typically, the first wire bond bonds a wire to a contact pad on the die and the second wire bond bonds a wire to a contact area on the substrate.
Typically, the method may include identifying further frequencies at which the amplitude of vibration is substantially zero at the energy generation plane and the mounting plane and substantially a maximum at the bonding plane, and performing a wire bond operation using one of the further identified frequencies.
In accordance with a second aspect of the present invention, wire bonding apparatus comprises an ultrasonic transducer comprising an ultrasonic energy generation device, an ultrasonic vibration amplifying device coupled at one end to the ultrasonic wave generation device, a bonding tool coupled to the opposite end of the ultrasonic vibration amplifying device, and a mounting structure located between the ends of the ultrasonic vibration amplifying device; and a signal generator having an output coupled to the energy generation device; the signal generator being adapted to generate at least one of a first and a second ultrasonic frequency to operate the energy generation device at the first or the second ultrasonic frequency, the first ultrasonic frequency being different from the second ultrasonic frequency.
Preferably, the apparatus further comprises a controller coupled to the signal generator to control which of the first and the second ultrasonic frequencies is generated by the signal generator.
Preferably, the apparatus also comprises feedback means coupled to the controller, the controller controlling the ultrasonic frequency generated by the signal generator in response to feedback signals received from the feedback means.